1. Field of the Invention
The present invention relates to a voltage-controlled piezoelectric oscillator which employs an inverting amplifier (inverter), and more particularly to a piezoelectric oscillator which features improved linearity of frequency change when the applied control voltage is in the low-voltage range.
2. Description of the Related Art
Voltage-controlled piezoelectric oscillators which employ piezoelectric vibrators and to which a control voltage is applied from the outside are widely used in various electronic apparatuses for obtaining a frequency signal which changes according to the control voltage in the vicinity of the resonance frequency of the piezoelectric vibrator. The control voltage is also used to compensate the temperature-frequency characteristic of the piezoelectric vibrator, whereby a fixed frequency signal can be obtained regardless of the ambient temperature. Quartz-crystal units are widely used as the piezoelectric vibrators, but ceramic vibrators may also be used in addition to crystal units.
FIG. 1 shows the construction of a voltage-controlled crystal oscillator which employs a crystal unit as the piezoelectric vibrator. In this oscillator, inverter 1 composed of, for example, a CMOS (complementary metal-oxide-semiconductor) circuit is used to amplify the resonance frequency component in the oscillation closed loop. Capacitors 2 and 3 are inserted between the input of inverter 1 and ground potential and between the output of inverter 1 and the ground potential, respectively. In addition, feedback resistor 4 is connected between the input and output of inverter 1. A series circuit composed of crystal unit 5, variable-capacitance diode 6, and capacitor 7 is further connected in parallel with feedback resistor 4. Return resistor 9 is provided between the ground potential and the connection point of the anode of variable-capacitance diode 6 and capacitor 7. Control voltage Vc for varying the oscillation frequency is applied by way of resistor a to the cathode of variable-capacitance diode 6. In this oscillator, oscillation output F is obtained from the output terminal of inverter 1.
In this oscillator, feedback resistor 4 is connected between the input and output of inverter 1 and a high level of gain is obtained by inverter 1, and the oscillation output therefore has a large amplitude which nearly rivals the power supply voltage which is supplied to inverter 1.
Regarding the uses of this oscillator, a number of applications exist in which an oscillation output of such great amplitude is preferable, but when constituted as a voltage-controlled oscillator, there is the problem that the high-frequency current which flows to the crystal unit or piezoelectric vibrator becomes excessive. In other words, there are the problems that, when the high-frequency current which flows through a crystal unit or piezoelectric vibrator becomes large, not only does the amount of frequency change become smaller when the capacitance of the variable-capacitance diode is changed, but the change in the oscillator frequency becomes non-linear with respect to change in the control voltage when the control voltage is within the range of a relatively low voltages.
It is an object of the present invention to provide a voltage-controlled piezoelectric oscillator employing an inverter which can easily obtain a high level of frequency change and excellent frequency linearity even when the control voltage is in a low voltage range.
The object of the present invention is achieved by a voltage-controlled piezoelectric oscillator which includes: an inverter having an input and output; a feedback resistor connected between the input and output; a first capacitor inserted between the input and the ground potential; a second capacitor inserted between the output and the ground potential; a series circuit which is composed of a piezoelectric vibrator, two variable-voltage capacitance elements, and a third capacitor, each variable-voltage capacitance element having a first terminal and a second terminal, the first terminals being connected to each other; a control terminal for applying a control voltage by way of a resistor to the common connection point of the first terminals of the variable-voltage capacitance elements; and return resistors inserted between the ground potential and the second terminals of variable-voltage capacitance elements.
In the present invention, the variable-voltage capacitance elements are components such as variable-capacitance diodes, and the piezoelectric vibrator is a component such as a quartz-crystal unit. When the variable-voltage capacitance elements are variable-capacitance diodes, the first terminals are, for example, the cathodes of the variable-capacitance diodes; and the second terminals are, for example, the anodes of the variable-capacitance diodes.
The present invention can obtain a voltage-controlled piezoelectric oscillator which can broaden the variable range of the oscillation frequency, and moreover, which can of course obtain good linearity in the range of relatively high voltages of the control voltage and that can further obtain good linearity in the range of relatively low voltages in which the maintenance of linearity was difficult in the prior art.